PURPOSEThis study determined the prenatal and postnatal factors associated with complications and prognosis in premature infants with leukemoid reaction.METHODSWe retrospectively reviewed the medical records of premature infants with gestational ages <37 weeks and low birth weights (<2,500 g) who were admitted immediately after birth to the neonatal intensive care unit at the Dongguk University Ilsan Hospital between June 2005 and July 2006. A leukemoid reaction was defined as an absolute neutrophil count (ANC) >30,000/mm3. The infants who had leukemoid reaction comprised the study group, while the remainder of infants made up the control group. The relationships between maternal and neonatal variables and ANC were studied.RESULTSLeukemoid reaction was detected in 3.1% of the study infants (8 of 252). Factors more frequently associated with infants with leukemoid reaction were as follows: maternal chorioamnionitis, high levels of maternal and infant C-reactive protein, gestational age <37 weeks, birth weight <2,500 g, low Apgar score, prolonged ventilator support, and a high incidence of bronchopulmonary dysplasia (BPD). However, there were no significant differences with respect to the antenatal usage of steroids, the incidences of patent ductus arteriosus, necrotizing enterocolitis, intraventricular hemorrhage, retinopathy of prematurity, and mortality between the two groups.CONCLUSIONLeukemoid reaction in premature infants was associated with chorioamnionitis and high levels of serum C-reactive protein in mothers and infants, and BPD in infants. These findings suggest that leukemoid reaction is secondary to inflammation caused by infection.